The present invention relates to a biaxially oriented polyester film and a capacitor using the same. More particularly, it relates to a biaxially oriented polyester film for a capacitor and a film capacitor using the same, which film has excellent adhesion properties for deposited metal, and is capable of providing high-level electrical properties and moist heat resistance to capacitor.
Polyester films represented by polyethylene terephthalate are popularly used as a film of magnetic tape, packaging film and makeup film because of their excellent mechanical and electrical properties, high heat resistance and chemical resistance in well balanced and also excellent cost performance. Miniaturization of electrical devices in recent years has intensified the request for reducing the size of a film capacitor comprising polyester film and deposited metal, and the demand of such capacitor increases more and more.
Incidentally, in the above polyester film for capacitor, the following excellent electric properties are required.
(1) The dielectric strength property is excellent and there is no insulation fault. In case where the thickness of the film is thin, the insulation property is deteriorated by a foreign substance in the film or thickness uniformity. PA1 (2) Dielectric properties such as dielectric constant and dielectric loss are excellent.
Especially, by the reduction of the film thickness attended with the request of miniaturization of capacitor, the property requirements for the polyester film used as dielectric are very severe. Further, an improvement of the property of polyester film is required under consideration of an influence by the coating layer.
Regarding the above-mentioned electric properties, it has been required to maintain good electric properties and to little change in the electric properties in the wide temperature range of an ordinary temperature to a high temperature. Namely, it is necessary that the electric properties is stable under the condition of high temperature and humidity for a long period of time, that is, it is required that the film has heat and humid stability of the electric properties.
In order to satisfy the above electric property requirements, a method of using a polyester film having a coating layer as a base film for capacitor has been proposed. However, there are some problems in this method. In case where the coating thickness is very small in.comparison with the polyester film thickness, the coating layer gives little influence to electrical properties of the polyester film and no problem arises. However, when the polyester film thickness is reduced, it becomes necessary to give considerations to the influence of the coating layer since the ratio of the thickness of the coating layer to that of the whole sheet is correspondingly enlarged.
Further, in order to attain the high capacitor property, it is required to consider the influence of heat-damage of the base film during metallization operation. Namely, it is required that even if metallization operation for the base film is carried out in severe heat condition, the film maintains good electric properties.
It is also notable as one of causes deteriorating the electric properties that by the dimensional change of the film caused by the thermal influence during in the capacitor production process, mechanical and electrical contact of the deposited metal with terminal electrodes (usually, metallicon) becomes inferior. This dimensional change of the film causes an increase of dielectric loss to give adverse effect to the capacitor properties.
The conventional polyester film for capacitor having the coating layer is not necessarily excellent in dielectric loss and alternative current withstand voltage, and further, by the continuous using, the electrostatic capacity of the capacitor is remarkably reduced.
By the present inventor's earnest studies for solving the above problems, it has been found that when the polyester film having a coating layer has a specific dimensional stability, the excellent electric properties and long-time stability against moist heat in the film are satisfied simultaneously and the excellent properties as a dielectric for capacitor are shown. The present invention has been based on this finding.